In many situations, there is a need for indicative markings on a product, for example, traceability markings indicating the origin of the product, or promotional markings. Such indicative markings can be provided by non-mechanical, i.e. non-contact, marking techniques, such as laser engraving or ink jet printing. These non-contact techniques provide for flexibility in production and high production speeds. Often, the markings must be precisely located in a specific area of the product. To this end, the marking equipment must be accurately controlled in providing the markings. Further, the material to be marked must be carefully positioned during the marking process. This is often inconsistent with high production speeds.
One area with a need for high production speeds is the beverage can industry, for example in the production of can ends. Typically, a production line for can ends has a capacity of 2,000 ends per minute. A conventional production line is disclosed in a brochure entitled “This is PLM Fosie” issued by Applicant's Swedish company PLM Fosie AB in the mid nineties. In a first production stage of such a production line, a thin metal strip, preferably a 0.23-mm-thick aluminium strip, is indexed into a tab forming unit in which the strip is punched and stamped to form opening tabs or opener rings integrated with the strip. In a second production stage, circular shells for forming the can ends are die cut from a thin metal sheet, preferably a 0.23-mm-thick aluminium sheet. Each shell is scored for opening, and a rivet for attachment of the tab is formed at the center of the shell. In a third production stage, the strip with the integrated tabs is joined with the circular shells in an attachment station, in which the tabs are separated from the strip and attached to the shells by riveting. A finished can end is achieved when the tab is fastened to the shell.
There is need for indicative markings on the tabs. Such markings could be provided by marking the strip before it is fed into the tab forming unit, in which the thus-marked strip is formed into marked tabs. However, due to the indexing motion of the strip into the tab forming unit, the strip will swing and jump in all directions on its way to the tab forming unit. Thus, to control the position of the strip, a guiding device should be arranged in the area of the marking operation. This guiding device should allow for careful positioning of the strip, but should not interfere with the intermittent progression of the strip into the tab forming unit. Also, stretching of the strip should be avoided, and friction should be minimized. Typically, the strip should be positioned with a precision of at least about 5-15 μm in the lateral, or transverse, direction, when providing markings on the surface of the tabs. The vertical position of the strip should also be carefully controlled within the focal region of marking equipment, typically with a precision of at least about 0.1-0.2 mm.